Does tcpdump show nothing for my C ++ application?

Question

If I run:

iperf -s -u -B 224.0.31.155

and run

sudo tcpdump -ni any 'host 224.0.31.155'

tcpdump is capable of capturing something:

tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 65535 bytes
15:49:15.334484 IP [some ip].14386 > 224.0.31.155.14386: UDP, length 1364
15:49:15.334728 IP [some ip].14386 > 224.0.31.155.14386: UDP, length 1374
15:49:15.375026 IP [some ip].14386 > 224.0.31.155.14386: UDP, length 1058
15:49:15.375184 IP [some ip].14386 > 224.0.31.155.14386: UDP, length 832

However, if I kill my iperf process above and then run a C ++ application that also joined the same group and binds the same port, tcpdump no longer sees traffic.

Here is a snippet:

struct sockaddr_in mc_addr; /* socket address structure */
struct ip_mreq mc_req; /* multicast request structure */
unsigned int from_len = sizeof(mc_addr); /* source addr length */

/* construct a multicast address structure */
memset(&mc_addr, 0, from_len);
mc_addr.sin_family = AF_INET;
inet_aton(mcastGroup.c_str(), &mc_addr.sin_addr);
mc_addr.sin_port = htons(port);

/* bind to multicast address to socket */
if (bind(s, (struct sockaddr *) &mc_addr, sizeof(mc_addr)) < 0) {
    std::cerr << "failed to bind to the port " << port << "|error="
            << strerror(errno) << std::endl;
    throw;
}
/* construct an IGMP join request structure */
mc_req.imr_multiaddr.s_addr = inet_addr(mcastGroup.c_str());
mc_req.imr_interface.s_addr = htonl(INADDR_ANY);

/* send an ADD MEMBERSHIP message via setsockopt */
if ((setsockopt(s, IPPROTO_IP, IP_ADD_MEMBERSHIP, (void*) &mc_req,
        sizeof(mc_req))) < 0) {
    std::cerr << "failed to set socket option to request for membership"
            << std::endl;
    throw;
}

Tcpdump details:

$ tcpdump --version
tcpdump version 4.1-PRE-CVS_2012_03_26
libpcap version 1.4.0

I just tested it on one of my production servers and showed the same behavior, but I see that my C ++ application is correctly processing the data.

Any idea what is going on?

Answer 1

, . .

UDP-, . .

, , INADDR_ANY Linux. . bind() UDP- Linux. IP- . IP- , IP-.

, mc_req.imr_interface, ip_mreq. , mc_req.imr_address, , , , .

Answer 2

IGMP . , IGMP, . , .